If you keep track of how many "virtual servers" your cluster has, and take into account the fact that the text obviously implies "per server" (let's not play dumb simply to be able to spit on the article), the math works correctly:
- I have x servers. I optimize to 1/10th of the time. I still have x servers but now have (1+9)x = 10x virtual servers.
- Now, I optimize again to 1/10th of the time. I have 10x virtual servers, so (1+9)(10x) = (1+99)x = 100x virtual servers.